The present invention relates in general to the semiconductor technology and more particularly to semiconductor accumulation-mode power devices with charge balance structures integrated therein.
FIG. 1 shows a cross section view of a conventional trenched-gate accumulation-mode vertical MOSFET 100. MOSFET 100 is an n-channel transistor with a gate terminal formed inside each of trenches 102, n-type regions 112 between trenches 102 (hereinafter referred to as the channel regions), a drift region 106, and an n-type drain region 114. Unlike enhancement-mode transistors, accumulation-mode transistor 100 does not include a blocking (p-type in this example) well or body region inside which the conduction channel is formed. Instead a conducting channel is formed when an accumulation layer is formed in channel regions 212. Transistor 200 is normally on or off depending on doping concentration of channel regions 212 and doping type of the gate electrodes. It is turned off when channel regions 112 are entirely depleted and lightly inverted. Because no inversion channel is formed, the channel resistance is eliminated thus improving the transistor power handling capability and its efficiency. Further, with no pn body diode, the losses in synchronous rectification circuits attributable to the pn diode are eliminated.
A drawback of accumulation transistor 100 is that drift region 106 needs to be lightly doped to support a high enough reverse bias voltage. However, a lightly doped drift region results in a lower on-resistance and lower efficiency. Thus, there is a need for an accumulation-mode transistor with low on-resistance, high blocking capability, and improved efficiency.